Detergent compositions have long employed materials, known as "builders", to improve the detergency of soaps and synthetic detergents by actively chelating alkali metal cations which are normal components of "hard" tap water. Such builders have been found to affect, for instance, soil suspension, emulsification of soil particles, solubilization of water-insolubles, and inactivation of various mineral constituents present in a detergent system. Many materials useful as builders have been proposed, and their effects are known. See, e.g., U.S. Pat. No. 3,852,213, U.S. Pat. No. 3,950,260, U.S. Pat. No. 4,182,718, and U.S. Pat. No. 4,440,646 (all incorporated herein by reference).
Recently, however, the attention of detergent manufacturers and researchers has turned to the role of heavier metal cations, i.e., transition metal cations and particularly iron, in the formation of stain complexes on fabrics and other surfaces. It has been observed that these multivalent transiton metal cations, particularly iron (Fe.sup.+++), enhance the binding of the components of many stains to substrates, and breaking up the cation-enhanced bonds is an effective approach to stain removal. Therefore, there is a strong need for the discovery of new materials that are effective as chelating agents for transition metal cations, are easy to prepare, and can be added to detergent compositions in economical amounts to boost stain-removing power.
It has now been discovered that certain cyclic N-hydroxyimides derived from citric acid esters are active transition metal ion chelants, particularly with respect to iron (Fe.sup.+++), making them attractive as additives for detergent compositions to aid in stain removal. In addition, the N-hydroxyimides of the present invention are believed to be broken down in freshwater systems to citric acid, which is well known to be biodegradable.